For the past three decades, biotelemetry has assisted many researchers and clinicians in obtaining physiological information from both patients and animals. With the development of new electronic, communication, battery, and material technologies, the capabilities of biotelemetry systems have expanded, bringing increased performance in the form of longer implantation times, greater channel counts, smaller sizes, and more robust communication.
For a majority of medical applications using biotelemetry implants, it has sufficed to monitor only a few channels of slowly varying DC levels such as pressure, temperature, ion concentration, etc. or small bandwidth signals with bandwidths typically ranging from 100 Hz to 5 kHz per channel such as EKG, EEG, EMG, etc. To date, however, biotelemetry systems have been unable to provide the throughput necessary for certain applications, such as cardiac mapping or high-bandwidth multichannel neural recording in which channel rates in excess of 1 Mbits/sec are often required, thus mandating large amounts of energy to power the implant. For long term studies, the energy requirement becomes even more prohibitive.
Researchers are currently searching for data collection systems that can maximize usage of developing, high-bandwidth sensor systems. Such sensor systems include flexible plastic substrate-based biosensor arrays for biopotential recording, and silicon-based micro-electrode arrays for neural recording.
Also of interest is the ability to collect physiological information from a variety of locations within a subject. This requires a network of sensors placed throughout a region under study. In cardiac mapping, for example, several electropotential arrays may be required at different ischemic or infarcted areas of a heart in order to simultaneously monitor electrical activity during a cardiac event. A desirable implementation for this network has each sensor as a separate telemeter, thereby eliminating the need to interconnect wires among the sensors. The elimination of these wires significantly reduces overall implant bulk and complexity while facilitating implantation.
A fundamental difficulty in developing a high-bandwidth biotelemetry system pertains to implant power consumption. In contrast to low-bandwidth systems, a high-bandwidth system must transmit many more pulses in a given time-period, thus depleting the power source much faster. In addition, the electronics required to sample, process, and encode the sensor data will also draw more energy as the aggregate bandwidth increases. The increased power demands require the use of larger implant batteries or alternative power sources. A popular widely known alternative to relying exclusively on batteries to power an implant is Inductive Power Transfer (IPT).
Inductive Power Transfer uses an AC-energized coil to create a magnetic field that couples with a receiving coil of an inductively powered device. The induced signal appearing at the output of the inductively powered device coil is then rectified and filtered to create a relatively constant DC power source. The "loosely-coupled transformer" link provides a means of eliminating and/or recharging inductively coupled biomedical implant batteries or capacitors. This technique has been used not only for biotelemetry devices, but also for artificial hearts, ventricular assist devices, various forms of neural stimulators, and battery recharging.
What is needed is a system which can accurately target arbitrarily oriented inductively powered devices in order to provide power to, and communicate at high data rates with, the arbitrarily oriented inductively powered devices.